As advised by the "Rapid Application Development with Mozilla" book by
Nigel McFarlane.
Indeed, a tag sole any binding when we use attribute like "display:none"
because a XBL binding requires a frame (styleable area).
Fixes #28

exchangecalendar <4.0 did a test on existence of an element with id
"todo-entrydate" to decide whether it should display or hide the
HTML content editor.
(this code was in the onLoad() function of calendar-event-dialog.js)
Because of the name of the ID, I misunderstood the test and thought
the HTML editor has to be displayed only for tasks.
This commit fixes the dialog display.
The next one will fix the content saving.